The present invention relates generally to magnetic random access memory (MRAM) devices, and, more particularly, to a method and structure for forming slot via bitlines for MRAM devices.
Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile random access memory technology that could replace the dynamic random access memory (DRAM) as the standard memory for computing devices. The use of MRAM as a non-volatile RAM would allow for “instant on” systems that come to life as soon as the system is turned on, thus saving the amount of time needed for a conventional PC, for example, to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory element (also referred to as a tunneling magneto-resistive, or TMR device) includes a structure having ferromagnetic layers separated by an insulating non-magnetic layer (barrier), and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, the magnetic moment of one magnetic layer (also referred to as a reference layer) is usually maintained in a preassigned direction, while the magnetic moment of the magnetic layer on the other side of the tunnel barrier (also referred to as a “free” layer) may be switched during operation between the same direction and the opposite direction with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment of the free layer adjacent to the tunnel junction are also known as “parallel” and “antiparallel” states, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments therebetween.
Depending upon the magnetic state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistance values in response to a voltage applied across the tunnel junction barrier. The particular resistance of the TMR device thus reflects the magnetization state of the free layer, wherein resistance is typically “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows a MRAM device to provide information stored in the magnetic memory element (i.e., a read operation). There are different methods for writing a MRAM cell; for example, a Stoner-Wohlfarth astroid MRAM cell is written to through the application of fields to exceed a critical curve or stability threshold, in order to magnetically align the free layer in a parallel or antiparallel state. The free layer is fabricated to have a preferred axis for the direction of magnetization called the “easy axis” (EA), and is typically set by a combination of intrinsic anisotropy, strain induced anisotropy, and shape anisotropy of the MTJ.
A practical MRAM device may have, for example, a cross point cell (XPC) configuration, in which each cell is located at the crossing point between parallel conductive wordlines in one horizontal plane and perpendicularly running bit lines in another horizontal plane. This particular configuration is advantageous in that the layout of the cells helps to increase the array cell density of the device. However, one difficulty associated with the practical operation of a cross-point MRAM array relates to the sensing of a particular cell, given that each cell in the array is coupled to the other cells through several parallel leakage paths. The resistance seen at one cross point equals the resistance of the memory cell at that cross point in parallel with resistances of memory cells in the other rows and columns, and thus can be difficult to accurately measure.
Accordingly, MRAM devices are also fabricated with a field effect transistor (FET) based configuration. In the FET-based configuration, each MRAM cell includes an access transistor associated therewith, in addition to an MTJ. By keeping the access transistors to cells not being read in a non-conductive state, parasitic device current is prevented from flowing through those other cells. The tradeoff with the FET-based configuration versus the XPC-based configuration is the area penalty associated with the location of the access transistors and additional metallization lines. In a conventionally formed FET-based MRAM device, the MTJ is typically formed over a conductive metal strap that laterally connects the bottom of the MTJ to the access FET (through a via, metallization line and contact area stud). A metal hardmask layer or via is then formed on the top of the MTJ that, in turn, is coupled to an upper metallization line.
Because of the continuing trend of decreasing device ground rules and smaller wiring sizes, the scaling of MRAM devices becomes extremely difficult due to the current-carrying restrictions on very narrow wires used for switching the state of the MRAM cells. Ferromagnetic liners around the switching wires have been used to focus the switching fields on the MTJs, however they are expected to be less effective as wire sizes shrink. The scaling to lower operating voltages makes the problem even worse, as even lower resistance wires are needed to pass the same amount of current. Accordingly, it would be beneficial to devise a process that utilizes conductors of lower resistance to pass larger currents for switching MRAM devices, and to devise a process that further locates the centroid of the switching current closer to the MTJ so as to generate larger switching fields at the MTJ for a given switching current.